Since its inception in 1996, the CO₂ injection operation at Sleipner has been monitored by 3D time-lapse seismic surveys. Striking images of the CO₂ plume have been obtained showing a multi-tier feature of high reflectivity. In the medium to longer term, the topmost layer of CO₂, accumulating and migrating directly beneath the topseal, is the main determinant of storage site performance. Fortunately it is this topmost layer that can be most accurately characterized, its rate of growth quantified, and CO₂ flux arriving at the reservoir top estimated. The latter is mostly controlled by pathway flow through thin intra-reservoir mudstones. This has increased steadily with time, suggesting either that pathway transmissivities are increasing with time and/or the pathways are becoming more numerous. Detailed 3D history-matching of the topmost layer cannot easily match the observed rate of spreading. Isotropic permeabilities result in a stronger radial component than observed and a degree of anisotropic permeability, higher in a north–south direction, is possible. The main contributor to the mismatch, however, is likely to be small but significant uncertainty in the depth conversion. Irrespective of uncertainty, the observed rate of lateral migration seems to require very high permeabilities, and is, moreover, suggestive of a topseal which behaves like a ‘hard’ impermeable flow barrier. Detailed studies such as this will provide important constraints on longer term predictive models of plume evolution and storage performance which are key regulatory requirements.